1. Field of the Invention:
This invention relates to a photometric device for a camera arranged to measure light by means of a photometric element consisting of a plurality of minutes photo-electric conversion elements.
2. Description of the Prior Art:
Heretofore varied kinds of photometric devices have been proposed for single-lens reflex cameras. They include, for example, a device arranged to have a light flux guided to a photometric element by a light splitting member such as a micro beam splitter formed at a condenser lens and another device arranged to condense light via a Fresnel lens which is provided on a light flux exit surface of a pentagonal roof type prism. However, they have presented a problem that there is an excessively adverse effect of reversely incident light (hereinafter will be called a back incident light) which comes from a view finder.
FIGS. 1 and 2 of the accompanying drawings show examples of the above-stated conventional photometric devices. Referring to FIG. 1 which is a sectional view of the optical system of a single-lens reflex camera, a light flux coming from an object to be photographed passes through a photo-taking lens 1 and is reflected upward by a reflex mirror 2. The reflected light flux is split by a light splitting member 4 via a focusing plate 3 into a light flux incident on a photometric element 5 and another light flux coming to an eyepiece 8 via a condenser lens 6 and a pentagonal roof type prism 7. In this case, a back incident light which is an external light incident on the eyepiece 8 comes also to the photometric element 5 via the pentagonal roof type prism 7, the condenser lens 6 and the light splitting member 4. It has been thus hardly possible with this device to perform an accurate photometric operation.
In the case of the conventional device shown in FIG. 2, a light flux coming from an object through the photo-taking lens is reflected by a reflex mirror 2. The reflected light comes through a focusing screen 9 and then is refracted by a pentagonal roof type prism 7 before it reaches an eyepiece 8. According to this photometric arrangement, the incoming light flux is arranged to be condensed on a photometric element 5 by a Fresnel lens 10 which is provided on the exit surface of the prism 7 with the element 5 disposed in a position corresponding to the exit point of the lens 10. In the case of FIG. 2 also, a back incident light from a view finder, i.e. an external light incident on the eyepiece, is reflected by a reflex surface of the pentagonal roof type prism 7 toward the Fresnel lens 10 and thus comes to the photometric element 5. Therefore, it has been also impossible to accurately perform a photometric operation with that device.
Meanwhile, Japanese Laid-Open patent application No. SHO 58-168039 has disclosed a photometric device. In this instance, a photo-electric conversion layer which includes an amorphous silicon photo diode is formed on a light-transmissive or -reflective surface of an optical member of either the photo-taking optical system or the view-finder optical system of a camera. In other words, this patent application proposes to form such a photo-electric conversion layer on the surface of a reflex mirror of a view-finder optical system, or on a surface of a pentagonal roof type prism, or on a focusing screen. However, the proposed method does not solve the problem of the conventional device shown in FIGS. 1 and 2, because: The above-stated back incident light from the view-finder likewise comes to the photo-electric conversion layer to prevent accurate light measurement.